1. Field of the Invention
This invention relates generally to the structures and manufacturing methods of an anisotropic conductive film (ACF). More particularly, this invention relates to new structures and manufacturing processes of an ACF of improved resolution and reliability of electrical connection at a significantly lower production cost.
2. Description of the Related Art
Current technologies for manufacturing anisotropic conductive films (ACF) or Z-axis conductive adhesive film (ZAF) for interconnecting array of electrodes such as those in liquid crystal display interconnections, chip-on-glass, chip-on-film, flip chip bonding and flexible circuits applications, are still challenged by several major technical difficulties and limitations. For those of ordinary skills in the art, there are still no technical solutions to overcome these difficulties and limitations. The ACF or ZAF comprising conductive particles dispersed in the adhesive film allows electric interconnection in the Z-direction through the thickness of the ACF layer. But horizontally these conductive particles are spaced far enough apart so that the film is electrically insulating in the X-Y directions. ACF comprising electrically conductive particles formed of narrowly dispersed metal-coated plastic particles such as Au/Ni-plated cross-linked polymer beads were taught in, for example, U.S. Pat. Nos. 4,740,657, 6,042,894, 6,352,775, 6,632,532 and J. Applied Polymer Science, 56, 769 (1995). FIG. 1A shows a typical ACF comprising two release films, an adhesive and conductive particles dispersed therein. FIG. 1B shows an exemplary application of an ACF implemented for providing vertical electrical connections between a top and bottom flexible printed circuit (FPC) or chip on film (COF) packages. FIGS. 1C and 1D show the schematic drawings of some typical ACF applications in connecting electrodes and IC chips. To ensure good electric contacts between the electrodes disposed above and below the ACF, conductive particles formed with rigid metallic spikes extended out from the particle surface may be implemented. These rigid metallic spikes improve the reliability of electric connection between electrodes susceptible to corrosion by penetrating through the corrosive film that may be developed over time on the surface of the electrodes.
The first difficulty faced by the conventional technologies is related to the slow and costly processes commonly employed in the preparation and purification of the narrowly dispersed conductive particles commonly implemented as the anisotropic conductive medium. The second difficulty is related to the preparation of ACF for high resolution or fine pitch application. As the pitch size between the electrodes decreases, the total area available for conductive connection in the z-direction by the ACF also decreases. Increasing the concentration of the conductive particles in ACF may increase the total connecting area available for connecting electrodes along the z-direction. However, the increase in the particle concentration may also result in an increase in the conductivity in the x-y direction due to the probability of increase of undesirable particle-particle interaction or aggregation. The degree of aggregation of particles in a dry coating film in general increases dramatically with increasing particle concentration, particularly if the volume concentration of the particles exceeds 15-20%. An improved ACF was disclosed in U.S. Pat. No. 6,671,024 in which a predetermined number of conductive particles were uniformly sprinkled onto an adhesive layer by for examples airflow, static electricity, free falling, spraying, or printing. However, the concentration of the conductive particles that may be used in the processes is still limited by the statistic probability of particle-particle contact or aggregation.
Attempts to reduce the conductivity in the X-Y plane have been disclosed in prior art such as U.S. Pat. No. 5,087,494 (1992), in which a process of making conductive adhesive tape was disclosed by making a predetermined pattern of dimples of a low adhesive surface followed by filling each dimple with a plurality of electrically conductive particles optionally with a binder. An adhesive layer was then applied as an overcoat onto the filled dimples.
In U.S. Pat. No. 5,275,856 (1994), a similar fixed array ACF having an array of perforation was disclosed. Each perforation contains a plurality of electrically conductive particles in contact with an adhesive layer, which is substantially free from electrically conductive material. In either case, the dimple or perforation was filled with, for example, a conductive paste or ink comprising dispersed conductive particles such as silver and nickel particles, and each dimple or perforation contains a plurality of conductive particles. The resultant filled dimples or perforation is relatively rigid and not easy to deform during bonding. Moreover, the filling process often results in an under-filled dimple or perforation due to the presence of solvent or diluent in the paste or ink. A high electrical resistance in the bonding area and a poor environmental and physicomechanical stability of the electric connections are often the issues of this type of ACFs.
In U.S. Pat. Nos. 5,366,140 and 5,486,427, another type of fixed array ACF was disclosed. A thin, low melting metal film on a carrier substrate was cut through the metal layer into a predetermined grid pattern and heated to a temperature higher than the melting point of the metal layer. The metal is beaded up due to the high surface tension of the metal and form an array of metal particles on the substrate. The process relies on a sophisticated balance of surface tension of the metal and the adhesion at the metal/substrate interface. The metal beading process is very susceptible to the presence of impurity or contamination on the surface/interface. The size of the metal bead may be formed by this process is also relatively limited. The shape of the metal bead is also mainly semispherical. A spike on the bead surface is quite difficult to form. Moreover, the metal bead is not easy to deform during bonding and results in a high electrical resistance in the bonding area more often with a poor environmental and physicomechanical stability of the electric connections.
In U.S. Pat. No. 4,606,962, a process of forming ordered array of conductive particles was disclosed. The process includes the steps of rendering areas of an adhesive coating substantially tack-free followed by applying electrically conductive particles only onto the tacky area of the adhesive. In U.S. Pat. No. 5,300,340 (1994) an ACF was prepared by printing an array of conductive particles on a carrier web by for example a negative-working Toray waterless printing plate. In U.S. Pat. No. 5,163,837 an ordered array of connector was prepared by depositing an adhesive into the mesh of an insulating mesh sheet followed by applying discrete electric contacts of a size to fit and fill the mesh. In either the direct or direct printing of conductive particles on a substrate, the size of the printed dots tends to be relatively big and non-uniform and missing particles or aggregation of particles are problems for reliable connections. The resultant ACF is not suitable for fine pitch applications.
U.S. Pat. No. 5,522,962 (1996) taught a method of forming ACF by (1) coating electrically conductive ferromagnetic particles into recesses such as grooves of a carrier web (2) providing a binder, and (3) applying a magnetic field sufficient to align the ferromagnetic particles into continuous magnetic columns, the magnetic columns extending from a recess in the carrier web. The process may allow a better separation of conductive materials by aligning the particles into well separate columns. However, the mechanical integrity, conductivity, and compressibility of metallic columns are still potential problems for reliable connections.
A fixed array ACF was disclosed by K. Ishibashi and J. Kimbura in AMP J. of Tech., Vol. 5, p. 24, 1996. The manufacturing of ACF involves an expensive and tedious process including the steps of photolithographic exposure of resist on a metal substrate, development of photoresist, electroforming, stripping of resist, and finally overcoating of an adhesive. ACF produced piece-by-piece using this batch-wise process could be fine pitch. However, the resultant high cost ACF is not very suitable for practical applications partly because the compressibility or the ability to form conformation contacts with the electrodes and corrosion resistance of the electroformed metallic (Ni) columns are not acceptable and result in a poor electrical contact with the device to be connected to. A similar resin sheet comprising a fixed array of through holes filled with metal substance such as cupper with Ni/Au surface plating on the top surface was taught in U.S. Pat. Nos. 5,136,359 and 5,438,223 and was disclosed as a testing tool by Yamaguchi and Asai in Nitto Giho, Vol. 40, p. 17 (2002). The inability to form conformation contacts with electrodes remains to be an issue. It results in high electrical resistance in the bonding area and a poor long term stability of the electrical connection. Moreover, it's very difficult to form a spike on the top of the electroformed metal columns.
In U.S. Pat. Nos. 6,423,172, 6,402,876, 6,180,226, 5,916,641 and 5,769,996, a non-random array was prepared by coating a curable ferrofluid composition comprising a mixture of conductive particles and ferromagnetic particle under a magnetic field. The ferromagnetic particles are relatively small in size and are washed away later to reveal an array of conductive particles well separated from each other. The processes of using ferromagnetic particles and the subsequent removal of them are costly and the pitch size of the resultant ACF is also limited by the loading of the ferromagnetic particles and conductive particles. Conductive particles with a spike may be used in this prior art to improve the connection to electrodes with a thin corrosion or oxide layer, but the direction of the spike on the particles prepared by this process is randomly oriented. As a result, the effectiveness of the spikes in penetrating into the oxide surface is quite low.
FIG. 1E shows the schematic drawing (not to scale) of a cross sectional view of a non-random array of conductive particles with a more or less random distribution of spike direction prepared according to the methods of U.S. Pat. Nos. 6,423,172, 6,402,876, 6,180,226, 5,916,641 and 5,769,996. The presence of the spike improves the reliability and effectiveness of connection to the electrodes, particularly to electrodes that are susceptive to corrosion or oxidation. However, only the spikes directed toward the electrode surface are effective in penetrating into the oxide layer of the electrode.
Therefore, a need exits in the art to provide an improved configuration and procedure for the manufacturing of ACF, particularly those with improved pitch resolution and connection reliability particularly for those electrodes that are susceptive to oxidation or corrosion.